The practice of measuring end-tidal carbon dioxide during the administration of anesthesia, particularly regional anesthesia, has grown markedly in the past several years. The reasons that anesthesiologists have embraced this technique are described more fully in U.S. Pat. No. 5,335,656 which is incorporated herein by reference in its entirety.
The preferred nasal cannula used in this procedure is a cannula which insufflates the patient with oxygen through one nare of a cannula and separately samples the exhaled gases by drawing the exhaled gas from the other nare into a conventional carbon dioxide analyzer. The cannula is preferably provided with an internal wall or system in the face piece to keep the conduits separate for insufflation and sampling, however, separate lines can be used or even multiple nares for insufflation and sampling, though the latter device substantially increases the risk of gases mixing which can distort the readings for end-tidal carbon dioxide. It is preferred that two nares only are employed and that each nare performs only one function, i.e., insufflation or sampling into or from separate nostrils. Likewise, insufflation has normally been continuous, however, it could advantageously be intermittent which would further improve the end-tidal carbon dioxide measurement by insuring that gases being sampled were representative of exhaled gases undiluted by the other gases being insufflated. Most preferably, the intermittent insufflation is accomplished by the apparatus and method described in U.S. Pat. No. 5,626,131 which is incorporated herein by reference in its entirety. Other so-called demand insufflation devices which begin insufflation upon the start of inhalation can also be employed.
Normal nasal cannulae are designed with the nares having a slight inward curvature as they extend upward from the face piece. This is anatomically desirable and important for imparting the proper direction of insufflating gas into the nasal cavities. When the patient is in the upright sitting position or ambulatory, this is the most satisfactory design configuration. Conversely, problems can be encountered if the patient is horizontal or prone and tends to accumulate secretions in the nasal cavities. It can be a particularly vexing problem if either the insufflation or sampling nare becomes occluded during the use of the cannula for sampling and monitoring end-tidal carbon dioxide during the administration of anesthesia.
It is therefore an object of the present invention to provide a nasal cannula structure for sampling carbon dioxide which reduces or eliminates the incidence of occlusion of the tip of the carbon dioxide sampling nare during the removal of carbon dioxide by the sampling line connected to a monitoring device and/or a source of suction or vacuum.
It is also an object of the present invention to provide a nasal cannula for insufflating a patient with oxygen while accurately monitoring end-tidal carbon dioxide, that will continue to function properly for its intended purpose when either or both nares become occluded for any reason.
It is a further object to accomplish the foregoing objects with a minimum risk of distorting the end-tidal carbon dioxide readings from the sampled exhalation gases during the administration of anesthesia.
The foregoing objects and advantages are obtained by providing a nasal cannula structure that is adapted for insufflation and sampling, with additional holes or vents on the nares of the nasal cannula, preferably both anterior and posterior of one or both nares at a location proximate the entrance of the nasal passageways when the cannula is in use.